Extremely sensitive optical sensors have been constructed by exploiting an effect known as surface plasmon resonance (SPR). These sensors are capable of detecting the presence of a wide variety of materials in concentrations as low as picomoles per liter. SPR sensors have been constructed to detect many biomolecules including keyhole limpet hemocyanin, .alpha.-fetoprotein, IgE, IgG, bovine and human serum albumin, glucose, urea, avidin, lectin, DNA, RNA, HIV antibodies, human transferrin, and chymotrypsinogen. Additionally, SPR sensors have been built which detect chemicals such as polyazulene and nitrobenzenes and various gases such as halothane, trichloroethane and carbon tetrachloride.
An SPR sensor is constructed by sensitizing a surface of a substrate to a specific substance. Typically, the surface of the substrate is coated with a thin film of metal such as silver, gold or aluminum. Next, a monomolecular layer of sensitizing material, such as complementary antigens, is covalently bonded to the surface of the thin film. In this manner, the thin film is capable of interacting with a predetermined chemical, biochemical or biological substance. When an SPR sensor is exposed to a sample that includes a targeted substance, the substance attaches to the sensitizing material and changes the effective index of refraction at the surface of the sensor. Detection of the targeted substance is accomplished by observing the optical properties of the surface of the SPR sensor.
The most common SPR sensor involves exposing the surface of the sensor to a light beam through a glass prism. At a specific angle of incidence, known as the resonance angle, a component of the light beam's wavevector in the plane of the sensor surface matches a wavevector of a surface plasmon in the thin film. At this angle of incidence a very efficient energy transfer and excitation of the surface plasmon occurs in the thin film. As a result, the amount of reflected light from the surface of the sensor changes. Typically, the reflected light exhibits an anomaly, such as a sharp attenuation or amplification, and the resonance angle of an SPR sensor can be readily detected. When the targeted substance attaches to the surface of the sensor, a shift in the resonance angle occurs due to the change in the refractive index at the surface of the sensor. A quantitative measure of the concentration of the targeted substance can be calculated according to the magnitude of shift in the resonance angle.
SPR sensors have also been constructed using metallized diffraction gratings instead of prisms. For SPR grating sensors, resonance occurs when a component of the incident light polarization is perpendicular to the groove direction of the grating and the angle of incidence is appropriate for energy transfer and excitation of the thin metal film. As with prism-based sensors, a change in the amount of light reflected is observed when the angle of incidence equals the resonance angle. Previous SPR grating sensors have incorporated square-wave or sinusoidal groove profiles.
Another highly-sensitive sensor that has been recently developed is known as a "diffraction anomaly" sensor. Diffraction anomaly sensors include a substrate and a thin metal layer that are substantially the same as in an SPR grating sensor. In a diffraction anomaly sensor, however, a dielectric layer is formed outwardly from the metal layer and protects the metal layer from oxidation and general degradation. Typically, a sensitizing layer is formed outwardly from the dielectric layer. Diffraction anomaly sensors, like SPR sensors, exhibit a change in reflectivity, referred to as a diffraction anomaly, when exposed with a light beam at a particular angle of incidence. Unlike conventional SPR sensors, diffraction anomaly sensors exhibit a change in reflectivity for light polarized parallel to the grooves of the substrate. When a light beam has an angle of incidence equal to the diffraction anomaly angle for the sensor, the diffracted light beam propagates within the dielectric layer. In this manner, the dielectric layer acts as a waveguide and the controller readily detects a change in reflectivity. The thickness of the dielectric layer directly affects the diffraction anomaly. The effective index of refraction at the surface of the diffraction anomaly sensor changes in a manner similar to an SPR sensor when the diffraction anomaly sensor is exposed to a sample containing the targeted substance. Furthermore, the change in the diffraction anomaly angle is strongly dependent upon the amount of targeted substance present in the sample. In this manner, the diffraction anomaly sensor exhibits a shift in the anomaly angle that is comparable to an SPR sensor, even though the metal grating of the diffraction anomaly sensor is coated with a dielectric layer. Therefore, a quantitative measure of the targeted substance can be calculated by measuring the resulting shift in the anomaly angle.
In addition to individual sensors, there is considerable commercial interest in multiple-sensor systems that are capable of detecting a variety of targeted substances, such as certain odors, vapors, gases and other chemical species, in a surrounding environment or sample. By utilizing several sensors, such sensing systems are capable of simultaneously detecting several targeted substances. Other multiple-sensor systems utilize multiple sensors to recognize the presence of a single targeted substance. In this configuration, the burden of recognition does not lie upon a single sensor, but rests on the sensing system's ability to properly interpret and recognize output patterns of the multiple sensors. Due to the use of multiple sensors, conventional multiple-sensor sensing systems are typically extremely expensive. Furthermore, conventional sensing systems are inherently complicated and therefore are not very portable.
Current optical sensing systems that incorporate SPR sensors or diffraction anomaly sensors must precisely measure an absolute shift in the resonance angle in order to accurately calculate substance concentration. One inherent deficiency with this technique is that slight mechanical changes in the sensor affect the angle of incidence, thereby leading to false resonance shifts. Additionally, any slight deviation in the wavelength of the incident light may cause a shift in resonance angle and lead to faulty results. For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon understanding the present invention, there is a need in the art for an optical sensor having improved sensitivity and less susceptibility to system variations.